Optical wavelength-division multiplexing (WDM) combines multiple optical channels for parallel transmission through a waveguide. A multiplexer is used to combine the multiple optical channels. A demultiplexer is used to separate the multiple optical channels. An example of a demultiplexer is an arrayed waveguide grating. After separating the multiple optical channels, a photodetector is used to convert an optical signal of an optical channel into an electrical signal. Integrated circuits (“ICs”), especially silicon, have dominated the development of electronics and photonics. An example of a photodetector using Germanium on a silicon substrate is provided in PCT App. No. US2012/0408809, published as WO2012167282.